The control and adaptation of transmit parameters generally include issues such as power control and rate adaptation.
Power control is used in numerous wireless systems, such as cellular systems, to enable “efficient” communication without expending unnecessary power resources.
Many different power control mechanisms have been proposed. A good overview is given in reference [1], where existing power control methods are classified into two categories, fixed and variable rate power control.
Most power control schemes control the SIR (Signal-to-Interference Ratio) or CIR (Carrier-to-Interference Ratio) to certain target values.
Early work on fixed-rate power control was performed already in the 1960's, with the main objective to obtain the same quality on all links, so-called quality balancing or SIR balancing. The basic concept of SIR balancing was later enhanced, deriving an optimum power assignment for minimizing the outage probability in terms of finding the maximum achievable SIR that all links can simultaneously reach. Balancing for heterogeneous SIR targets has also been investigated.
Some work in this area has focused on developing practical SIR balancing algorithms, without the excessive effort of collecting the necessary information to a centralized controller. For this purpose iterative and distributed balancing algorithms were introduced. Common for these algorithms is the exclusion of background noise, which makes scaling of the power vector necessary. To alleviate this problem some extensions of the distributed algorithms with preset SIR targets and non-zero background noise have been proposed.
In the area of downlink power allocation for CDMA, cell powers may be allocated such that all users in the cell experience the same SIR during convergence to a preset target, taking into account a total sum power constraint. However, this may cause an entire cell to have insufficient quality. Another approach for power control in CDMA is to provide for constant received power at the base station. Unfortunately, this approach generally does not have any significant effect on the co-channel interference.
The so-called Minimum Power Assignment (MPA) problem includes base station selection for finding the lowest possible uplink power vector.
With regard to variable-rate power control, maximum achievable channel capacity has traditionally been a well-examined topic. The capacity regions for a fading uplink single-cell multiple access channel have been found, considering both delay tolerant and intolerant cases. Water filling has also been applied, for example in the time-domain for power and rate to achieve capacity over a fading channel. The results highlight a common characteristic of conventional throughput maximization, namely to allocate resources to good channels.
Greedy power control algorithms generally operate without any SIR targets, and assign high data rates starting with mobiles having high link gain. However, this type of power control is quite unstable and generally results in either maximum or zero power being used.
So-called truncated power control has been considered over fading channels with the objective to avoid loss of capacity when compensating for deep fades by decreasing transmission rate or power or both. A truncated rate adaptation scheme based on suspending transmission when the link gain is below some threshold has been suggested for traffic tolerating longer delays.
Rate adaptation is another example of control and adaptation of transmit parameters, where transmission parameters (normally modulation and coding scheme) used for communication are adapted in order to optimally exploit the potential of the communication medium, usually to provide high data transfer rates along with low bit error rates.
Early work on rate adaptation in DS-CDMA considered minimization of the total downlink transmitted power given constraints on individual user data rates. An alternative approach is to maximize the sum rate of an uplink of a single-cell system with the requirement that the energy to noise density per bit exceeds a minimum level for each link. It has also been suggested to maximize the sum rate in a CDMA system with the requirement that the energy to noise density per bit should be equal to a certain level for each link. Yet another suggested approach is to maximize the sum rate of an uplink in CDMA subject to a constraint on total received power where the sum of all received powers (noise and interference excluded) should be equal to a constant.
Although some of the prior art schemes may provide quite satisfactory performance in specific cases, the conventional power/rate control schemes generally leave room for considerable improvements.
Reference [2] relates to stream control in networks with interfering MIMO links. The throughput performance is targeted and the basic idea is to maximize throughput by introducing a constraint to limit the number of independent streams for each transmitting node in a network of interfering MIMO links.